Why brainy animals need more REM sleep after all

Why do donkeys snooze for just three hours a day, while hairy armadillos are knocked out for more than 20? Biologists have struggled to find any satisfactory explanation for the bewildering variation in how much different mammals sleep.

However, new studies that take evolutionary relatedness into account promise to revolutionise the field. In particular, one large study suggests that REM (rapid eye movement) sleep - during which the brain is highly active - may play a key role in intelligence.

Lab studies in humans already suggest that REM sleep is important for cognitive abilities such as consolidating memories - a good night's sleep - with plenty in the REM phase - can improve people's ability to remember what they have learned in the day by about 15%.

In other species, the evidence is less clear cut. If REM sleep helps learning, then mammals with more developed brains should presumably need more of it, but in the past no such relationship has been found.

Fatal flaw

One of the few biological functions that has been found to correlate with sleep patterns is metabolic rate. Animals with a relatively high metabolic rate for their body size seem to need more non-REM sleep, suggesting that catching extra Zs simply helps them conserve precious energy.

But John Lesku of the Max Planck Institute for Ornithology in Starnberg, Germany, believes these studies had a fatal flaw - researchers were comparing species without taking their evolutionary relationships into account.

He says that big differences in sleep patterns between evolutionary groups can swamp relationships found within those groups (see graph, below right).

So Lesku and his colleagues compiled studies looking at sleep patterns in 83 species of mammals - from opossums and sloths, to cows, beavers, macaques and people - and reanalysed the data using statistical techniques to account for their position on the evolutionary tree.

"These techniques are standard throughout biology," he says, "but for some reason they never permeated sleep research."

'Call to arms'

The results contradict previous work. Once evolutionary relationships were factored in, the data showed that animals with big brains for their body size need a significantly higher percentage of REM sleep - supporting a role in intelligence and cognitive function.

And species with high metabolic rates for their size needed less non-REM sleep, not more. This suggests that they don't sleep to conserve energy. Instead, animals with high metabolic rates may sleep less because they burn more calories, so have to spend more time foraging for food.

So this month he will publish a broader discussion of his results in Sleep Medicine Reviews, as a "call to arms" for the field. "It's to emphasise the necessity for these kinds of research," he told New Scientist. "Evolution does matter."

'Strongest signal'

Lesku's point of view is supported by another study by Isabella Capellini of Durham University, UK, and colleagues, who tested the strength of the influence that evolutionary relatedness has on sleep patterns - and found it to be highly significant.

"Sleep scientists have ignored the fact that sleep could be affected by evolutionary relationships," says Capellini. "But it's the strongest possible signal."

After taking evolution into account, Capellini's team found that ecological factors are more important than previously acknowledged. For example, species at risk of predation tend to sleep less. "Prey can't afford to sleep for longer," she says. "This indicates that if they still sleep at all, it must do something important."

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Table illustrating how sleep varies with brain size. For each animal, the table gives (a) how many hours of sleep it takes per day, (b) the percentage of that time it spends in REM sleep, and (c) the percentage of its total body mass taken up by its brain (Images: Wikimedia Commons)